HP - A 9
Sequences Of Operation
20 SEER Sequence of Operation
Heat Pump Heating Mode of Operation
Electronic Expansion Valve (EEV)
The EEV control board receives up to 5 analog signals from
the low voltage side of the main control board. 24 volt AC
power is sent from the J2 plug on the main control board to
J6/R of the EEV control board while the 24 volt common
connection to J6/B is made at a terminal strip located near the
bottom of the control box. When a heat pump system is first
powered up, an audible sound (soft ratcheting sound) may be
heard as the valve drives to the closed position.
When a heat pump system receives a demand message for
first stage heating capacity, in addition to the compressor
contactor and outdoor fan control signals, there is also a 24 volt
AC output from the main control board plug J3/1 (Y1) to J6/
Y1 of the EEV control board. There is a green STATUS
LED on the EEV control board that will flash rapidly to
indicate that a signal is being sent from plug J1 to the stepper
motor coil which will drive the EEV valve stem to a
predetermined position for first stage refrigerant flow control.
Superheat of 12 degrees (+/-2) is maintained by monitoring
suction line temperature and pressure. Necessary adjustments
to the rate of refrigerant flow are made via the position of the
EEV valve stem which is directly controlled by 12VDC pulse
signals from the J1 plug of the control board.
Suction line temperature is determined by thermistor input to
plug J10 of the EEV control board. Suction pressure is
determined by transducer input to plug J7 of the EEV control
board.
When a heat pump system receives a demand message for
second stage heating capacity, in addition to the compressor
contactor, outdoor fan control and EEV J6/Y1signals, there
is also a 24 volt AC output from the main control board plug
J3/Y2 to J6/Y2 of the EEV control board. The green
STATUS LED on the EEV control board will flash rapidly to
indicate that a signal is being sent from plug J1 to the stepper
motor coil which will drive the EEV valve stem to a
predetermined position for second stage refrigerant flow control.
Superheat of 12 degrees (+/-2) is maintained by monitoring
suction line temperature and pressure. Necessary adjustments
to the rate of refrigerant flow are made via the position of the
EEV valve stem which is directly controlled by pulse signals
from the J1 plug of the control board to the stepper motor
coil.
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Note: The EEV closes with every OFF cycle in the
heating mode of operation. During Defrost or cooling mode
of operation, the valve will drive to half open.
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Defrost Mode
During the heating cycle, the outdoor coil may become frosted
or iced-up which reduces system capacity. The main control
board determines when the system requires a defrost cycle
using Demand Defrost logic. Two thermistor inputs are used to
compare outdoor ambient temperature to coil temperature. The
Ambient Sensor is located below the control box where the
outside air temperature can be sensed and the Coil Sensor is
located in a copper well attached to one of the lower circuits of
the outside coil. The delta between these sensors is used to
calculate when the outdoor coil is frosted and needs to be
cleared.
Defrost initiation from first stage heating cycle
When the system is operating in first stage heating and the
control initiates a defrost cycle, the control simultaneously
de-energizes MS-LO to turn OFF CPR-LO, energizes MS-
HI to turn ON CPR-HI and de-energizes the PWM signal
to the outdoor fan motor. At the same time the control
energized the SOV solenoid coil to the cooling mode position.
The control also sends a demand message to the indoor unit
to run the blower at 2
nd
stage cooling speed and energize
auxiliary heat (if equipped). Auxiliary heat blower speed may
be higher than 2
nd
stage compressor blower speed and will
take precedence during defrost. The Y1 and Y2 LEDs will be
ON. The EEV will drive to ½ open based on the presence of
a 24 volt AC signal to J6/O.
Defrost termination to first stage heating cycle
When the control senses that the coil temperature has risen to
the programmed termination temperature or that the maximum
15 minute override has expired, the control will send a low
speed PWM signal to re-energize the outdoor fan motor.
There is a 12 second time delay (soft switchover) before de-
energizing the SOV solenoid coil back to the heating mode
position. The control will then de-energize MS-HI to turn OFF
CPR-HI and after a one minute Inter-Stage Delay, energize
MS-LO to turn ON CPR-LO. At the same time, a demand
message will be sent to the indoor unit to return to first stage
heating airflow and de-energize auxiliary heat. The Y1 LED will
be ON and the EEV control will send the necessary pulse
signals to the stepper motor coil returning the valve to first
stage heating position and begin monitoring superheat.
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